Extreme temperature regimes during the cool season

During the boreal cool season regional climate in the United States is strongly impacted by temperature extremes, heavy rain events and snow/ice storms. In particular, extreme temperature regimes such as cold air outbreaks significantly impact energy consumption and human safety (via exposure). We examine extreme temperature regimes over three distinct regions: the Midwest, Northeast Megalopolis, and Deep South, all of which include major urban areas. Analyses of regional statistical characteristics reveal that applying a wind chill criterion (versus a temperature-only criterion) results in an alteration of the severity ranking among cold air outbreak events. We overview regional long-term trends in the frequency and amplitude of extreme temperature regimes. We also quantify the statistical relationship between extreme temperature regimes and prominent natural modes of low frequency variability. The primary datasets for this portion of the study are the NCEP-NCAR and the ERA-40 reanalyses.

Concentrating on cold air outbreak events over the Deep South, we perform synoptic and dynamic diagnostic analyses of individual large amplitude events occurring during January 2004, April 2007 and January 2010. Our previous research indicates that the NASA-GMAO MERRA and NARR datasets are superior to NCEP-NCAR in characterizing the detailed structure of local thermal and moisture gradients associated with cold air outbreaks. Thus we choose to use the high resolution MERRA dataset in the second portion of our research. We first characterize the three-dimensional synoptic evolution of these events including the temporal relationship with large-scale teleconnection patterns. Dynamical diagnoses are then performed to infer (a) the large-scale dynamical triggers responsible for the lower tropospheric cold air surge during event onset and (b) the proximate source for the large-scale dynamical triggers, themselves. Our approach is based upon a potential vorticity dynamical framework. The first inference (a) is pursued using piecewise potential vorticity inversions while the latter inference (b) utilizes a combination of wave propagation diagnoses and local dynamical tendency budgets. Both local and remote wave sources are implicated as playing important roles in triggering cold air outbreaks over the Deep South.